Solid-state semiconductor lasers are important devices in applications such as optoelectronic communication systems and high-speed printing systems. Recently, there has been an increased interest in vertical cavity surface emitting lasers (“VCSELs”) although edge emitting lasers are currently used in the vast majority of applications. A reason for the interest in VCSELs is that edge emitting lasers produce a beam with a large angular divergence, making efficient collection of the emitted beam more difficult. Furthermore, edge emitting lasers cannot be tested until the wafer is cleaved into individual devices, the edges of which form the mirror facets of each device. On the other hand, not only does the beam of a VCSEL have a small angular divergence, a VCSEL emits light normal to the surface of the wafer. In addition, since VCSELs incorporate the mirrors monolithically in their design, they allow for on-wafer testing and the fabrication of one-dimensional or two-dimensional laser arrays.
VCSELs are typically made by growing several layers on a substrate material. VCSELs include a first mirrored stack, formed on the substrate by semiconductor manufacturing techniques, an active region, formed on top of the first mirrored stack, and a second mirrored stack formed on top of the active region. By providing a first contact on top of the second mirrored stack, and a second contact on the backside of the substrate, a current is forced through the active region, thus driving the VCSEL.
The active region is made up further of one or more quantum wells sandwiched between two spacer cladding regions. Inside the spacers, the active region is sandwiched by confining layers. The confining layers or regions are used to provide electrical confinement of minority carriers. By selecting the appropriate materials for the quantum well, the confining layers, and the barrier layers, a VCSEL generally may be grown or fabricated that generates light at a desirable, predetermined wavelength. For example, by using InGaAs quantum wells on GaAs substrates longer wavelength VCSELs can be produced. The use of InGaAs quantum wells, however, causes strain in the quantum wells. If the quantum wells are grown past their critical thickness, they relax by creating dislocations, and thus a poor quality active region.
The thickness of the various layers in the active region while not arbitrary have some flexibility within the constraints of the design and the process. The combined thickness of the spacers, the confining layers, the barriers and the active regions sandwiched by the mirrors must be such that a Fabrey Perot resonator is formed. The quantum wells should be positioned so that they are roughly centered at an antinode of the electric field. These two requirements define the spacer thickness in terms of the other layer thicknesses. The barrier layer thicknesses between the quantum wells need to be thick enough to adequately define the quantum wells, but thin enough that the quantum well positions are not excessively far from the antinode of the electric field. The thickness of the barrier layers at the boundaries of the quantum well regions have some flexibility. Optimally they need to be at least thick enough that the energy levels of each of the quantum wells is nominally the same. They can be thicker if material quality issues require this. The confining layers are often one in the same with the spacers or, as is shown in this work, can grade stepwise or continuously in the valence and conduction bands towards that of the barriers. Sometimes the confining layers and barrier layers are made from the same compositions, but this is not optimal for carrier confinement and is usually a compromise made for processing reasons.
The thickness of the quantum well is related by quantum mechanics to the well and barrier compositions, the desired emission wavelength, and the density of states. With a higher density of states narrower quantum wells can be optimally used.
What is needed in the VCSEL art are means to achieve long wavelength quantum wells normally fabricated on GaAs substrates. The present inventor has recognized that it would be advantageous to remedy the foregoing and other deficiencies in the prior art, and to facilitate the easier production of longer wavelength VCSELs. Accordingly, the present invention is described and presented as a novel methods and means to address the shortcomings currently found in the art.